This invention relates to a transponder, in particular a transponder for a contactless smart card. The term “transponder” refers here to an assembly comprising an antenna and an electronic circuit, in particular in the form of a chip, the energy required for operating the chip as well as data being received via the antenna formed as a coil. Such transponders are used in contactless smart cards, in labels provided on goods, in keys, particularly auto keys as immobilizers, and—usually incorporated in glass or porcelain capsules—in animal bodies for identification.
Said transponders are designed for unidirectional or bidirectional data exchange with an external device, referred to here as a reader. The reader radiates a high-frequency magnetic field via an antenna, the transponder taking energy from the magnetic field via a large-surface coil in fairly close proximity to the reader. At the same time as it absorbs energy, the transponder derives a clock signal. The structure and operation of such transponders is described extensively in the prior art, reference being made for example to US-C 5 841 123, the article “Kontaktlose Chipkarten” by Klaus Finkenzeller, Funkschau 19/98, pp. 40–43, and Klaus Finkenzeller, RFID-Handbuch, Carl Hanser Verlag, Munich/Vienna, 1999.
In order to facilitate the understanding of the invention, the basic features of a transponder will be explained in the following, being set forth later in more detail with reference to the description of the figures.
A reader radiates a high-frequency magnetic field of e.g. 13.56 MHz via an antenna. If the antenna of a transponder is located in said magnetic field, energy is coupled into the transponder due to the negative feedback between the two antennas. Electric power available to the transponder is proportional to magnetic field strength on the antenna. Field strength on the transponder is in inverse proportion to distance (1/x3) from the reader and thus varies very greatly in the working condition.
The microchip requires a constant supply voltage, operation being effected at a constant clock frequency. At very short distances between reader and transponder, the voltage induced in the transponder would exceed the required supply voltage if no countermeasures were taken. A shunt regulator is therefore switched in parallel with the load for converting excess power into heat to keep the supply voltage constant in the case of increased induced voltage in the transponder.